1. Field of the Invention
The present invention relates to a VoIP modem, and more specifically, to a VoIP modem capable of performing a silent redial with a public switched telephone network (PSTN).
2. Description of the Prior Art
With the growing popularity of high-speed Internet connections, it is now feasible for Voice over Internet Protocol (VoIP) phone calls to be made over the Internet. One main advantage of using the VoIP modem is VoIP phone calls are significantly less expensive than phone calls made over a public switched telephone network (PSTN). Unfortunately, there are times where quality of a VoIP connection suffers, and it is desired to switch the phone connection to the PSTN. Therefore, VoIP modems with PSTN backup capabilities have been developed. The VoIP modem is originally set in VoIP mode, and a user is allowed to switch it into PSTN mode to make a telephone call through PSTN. Additionally, it is desirable to use the PSTN in emergency situations due to its relative stability compared to VoIP connections. In this situation, the existing VoIP connection with a subscriber line interface circuit (SLIC) must be disconnected, and the VoIP modem establishes a new connection with the PSTN and automatically redials the number to PSTN.
Please refer to FIG. 1. FIG. 1 is a block diagram of a VoIP modem redial system 10 according to the prior art. The VoIP modem redial system 10 comprises a telephone set 12 and a mechanical relay 20 for connecting the telephone set 12 to a PSTN 22 or to a SLIC 14. A subscriber line interface circuit (SLIC) is used for interfacing analog telephone signals with a VoIP modem so that the voice data can be transferred through the VoIP network. In addition, the VoIP modem redial system 10 comprises an off-hook and ring detect circuit 18 connected to the PSTN 22 and to the telephone set 12 through the mechanical relay 20 for detecting a ring signal from the PSTN 22 and an off-hook status of the telephone set 12. Finally, a dual tone multi-frequency (DTMF) redial coupling circuit 16 is connected between the SLIC 14 and the PSTN 22 for passing redial signals from the SLIC 14 to the PSTN 22.
When the VoIP has a not ready status, the mechanical relay 20 will switch such that the telephone set 12 is connected to the PSTN 22, and the user can make phone call with the PSTN 22. Alternately, if the VoIP had a ready status, then the mechanical relay 20 will switch to connect the telephone set 12 to the SLIC 14 in order to utilize the VoIP service. A user can dial a predetermined code, for example “****”, to manually switch the relay 20 to connect the telephone set 12 to the PSTN 22.
Although making telephone calls over a VoIP network is less expensive, the voice quality of the VoIP phone calls is sometimes unstable, and, in some important situations, such as in an emergency, the user does not wish to encounter telephone service failure. In VoIP modems with PSTN backup capability, if the user dials in VoIP mode a telephone number (such as 911) which is pre-stored in the device, the number will be automatically redialed to PSTN to establish a phone call through the PSTN instead of through the VoIP network. That is, the VoIP modem redial system 10 will automatically switch the mechanical relay 20 to be connected to the PSTN 22 and send a redial signal to it. If the telephone set 12 uses tone-dialing, the SLIC will produce DTMF tone redial signals and send the signal to PSTN 22 through the DTMF redial coupling circuit 16. On the other hand, if the telephone set 12 uses pulse-dialing, the mechanical relay 20 will be controlled to quickly open and close to produce the correct dialing pulses. In this way, automatic redial is performed by the VoIP modem redial system 10.
In the above system, the redialing process is performed while the telephone set 12 is directly connected to the PSTN 22. This means that the user will hear redialing sounds associated with either the DTMF tone redial or the dialing pulse redial. However, these audible redialing sounds are distracting to the user of the VoIP modem redial system 10. The user can become confused when hearing these redialing sounds.